1. Field of the Invention
The present invention relates to an apparatus and method for testing pixels of a flat panel display.
2. Description of the Related Art
A flat panel display (FPD) is a display device for displaying electronic information. An example of the flat panel display (FPD) popularly used in the recent years is a liquid crystal display (LCD) using thin film transistors (TFTs). The liquid crystal display using the TFTs is used in a high-performance laptop computer or the like.
The configuration and operation of the liquid crystal display using the TFTs will be described below. As the basic structure, the liquid crystal display using the TFTs has a liquid crystal panel formed in such a manner that liquid crystal is poured between one glass substrate having TFTs and pixel electrodes formed thereon and the other glass substrate having counter electrodes formed thereon.
FIG. 5 is a schematic view showing a glass substrate on which TFTs and pixel electrodes are formed. In FIG. 5, a single glass substrate 11 has a plurality of panels 12 formed by a general process of manufacturing integrated circuits. Each of the panels 12 is constituted by a plurality of pixels 13 arranged in a matrix layout.
Each of the pixels 13 contains a pixel electrode 14, a storage capacitor 15, and a TFT 16. The pixel electrode 14 is made of a light-transmissive material. Generally, the pixel electrode 14 is made of ITO (indium tin oxide). The storage capacitor 15 has an electrode (hereinafter referred to as “Cs electrode”) to which a reference voltage of the pixel 13 is applied. The Cs electrode is grounded. That is, the reference voltage of each TFT 16 is set at a ground level. The TFT 16 functions as a switch. The TFT 16 has a gate electrode G supplied with a row selection signal LR for performing switching control, and a source electrode S supplied with a column selection signal LC as a data signal.
When a voltage VG is applied to the gate electrode G of the TFT 16 (i.e., a row selection signal LR is supplied) while a voltage VS is applied to the source electrode S of the TFT 16 (i.e., a column selection signal LC is supplied) at the time of driving of each pixel 13, the TFT 16 is switched on to increase a drain voltage VD. During that time, the storage capacitor 15 is charged to maintain the drain voltage VD until the next refreshing cycle. By repeating this process to all the pixels 13, the molecular arrangement of liquid crystal between the two glass substrates is controlled so that a two-dimensional image is displayed on the liquid crystal display.
For testing the glass substrate having TFTs and pixel electrodes formed thereon (hereinafter referred to as “TFT substrate”), there has been proposed a method for non-contact judgment of each pixel condition on the substrate on the basis of a voltage contrast technique using an electron beam (U.S. Pat. No. 5,982,190). The TFT substrate testing method using the voltage contrast technique has an advantage that this method is more inexpensive than any related-art testing method using a mechanical probing technique and is higher in testing speed than any optical testing method.
FIG. 6 is a drawing for explaining a TFT substrate testing method using a voltage contrast technique. This testing method is carried out in a high-vacuum chamber. A TFT substrate to be tested is carried into the high-vacuum chamber and disposed on a stage. In this condition, the TFT substrate is tested.
In FIG. 6, the testing apparatus includes an electron beam source 21, a secondary electron detector 24, and a signal analyzer (computer system or the like) 25. The electron beam source 21 generates an electron beam 22 with which each pixel 13 of the TFT substrate 11 is irradiated. The secondary electron detector 24 detects secondary electrons 23 generated on the basis of irradiation of each pixel 13 of the TFT substrate 11 with the electron beam 22. The secondary electron detector 24 outputs a signal indicating a waveform corresponding to the voltage waveform of the pixel 13 on the basis of the amount of the detected secondary electrons 23 and sends the signal to the signal analyzer 25. The signal analyzer 25 analyzes the output signal of the secondary electron detector 24 and checks each pixel condition, especially the presence/absence of a defect in the pixel and the details of the defect. The signal analyzer 25 includes a drive signal supply unit by which drive signals for driving each pixel 13 of the TFT substrate 11 is output through lines 26. Supply of the drive signals is synchronized with scanning of the TFT substrate 11 using the electron beam 22 generated by the electron beam source 21 in directions represented by the arrows S in FIG. 6.
The principle of the voltage contrast technique based on the amount of detected secondary electrons will be described below.
The amount of secondary electrons 23 emitted from each pixel 13 of the TFT substrate 11 depends on the polarity of the voltage of the pixel 13 of the TFT substrate 11. When, for example, a pixel 13 in the TFT substrate 11 is driven positively, secondary electrons 23 emitted on the basis of irradiation of the pixel 13 with an electron beam 22 are attracted to the pixel 13 because the secondary electrons 23 have negative electric charges. As a result, the amount of the secondary electrons 23 reaching the secondary electron detector 24 is reduced.
On the other hand, when a pixel 13 in the TFT substrate 11 is driven negatively, secondary electrons 23 emitted on the basis of irradiation of the pixel 13 with an electron beam 22 repel the pixel 13 because the secondary electrons 23 have negative electric charges. As a result, the secondary electrons 23 emitted from the pixel 13 reach the secondary electron detector 24 without reduction in the amount of the secondary electrons 23.
In this manner, the voltage signal waveform of the pixel 13 can be measured on the basis of the fact that the amount of the detected secondary electrons 23 emitted from the pixel 13 depends on the polarity of the voltage of the pixel 13.
In such a test, a method of molding a TFT substrate between a palette for holding the TFT substrate and a prober frame for applying a TFT driving voltage has been proposed in U.S. Pat. No. 6,765,203 to solve the problem that damage of the apparatus in a high-vacuum chamber is caused by breaking of the glass substrate in the high-vacuum chamber.
FIGS. 7A and 7B are perspective views showing a mode in which the TFT substrate is held between the palette for holding the TFT substrate and the prober frame for applying a TFT driving voltage. In this here, description is made on the case where a glass substrate having two panels 12 formed thereon is used as the TFT substrate 11, as shown in FIG. 7A.
In FIGS. 7A and 7B, the testing apparatus includes a palette 41 for holding the TFT substrate 11, and a prober 44 for supplying drive signals to each of pixels 13 on the TFT substrate 11.
The palette 41 has electrodes (not shown) formed on a stage 45 side and brought into contact with a power distribution portion 46 of the stage 45 opposite to the palette 41, a power distribution portion 42 provided on the prober 44 side and brought into contact with electrodes (not shown) of the prober 44, and a flexible circuit 43 to connect the two power distribution portions 46 and 42 to each other.
The prober 44 is shaped so that a glass portion except a panel 12-formed region of the TFT substrate 11 opposite to the prober 44 is covered with the prober 44. For example, in FIG. 7A, the prober 44 is shaped like a frame that surrounds the two panels 12 formed on the glass substrate. The prober 44 has electrodes (not shown) formed on the palette 41 side and brought into contact with the power distribution portion 42 of the palette 41, and prober pins (not shown) brought into contact with electrodes which are provided on the glass substrate opposite to the prober 44 and which are connected to the panels. The prober pins are connected to the electrodes formed on the palette 41 side of the prober 44 through wiring so that voltages from the electrodes formed on the palette 41 are applied to the electrodes provided on the glass substrate from the probe pins. Incidentally, the prober pins and the electrodes are mounted on the prober 44 through an electrically insulating material such as ceramics.
The palette 41 and the prober 44 are tightened to each other by fixing stutts such as bolts. Between the palette 41 and the prober 44 after assembling in this manner, the TFT substrate 11 is inserted and held (FIG. 7B).
For testing, the TFT substrate 11 held between the palette 41 and the prober 44 is carried into a high-vacuum chamber and sat on the stage 45. In the high-vacuum chamber, an apparatus control unit supplies a driving voltage to the electrodes of the prober 44 through the power distribution portion 46 of the stage 45 and the flexible circuit 43 and power distribution portion 42 of the palette 41 in a testing process. The prober pins of the prober 44 supply drive signals to the pixels 13 of the TFT substrate 11 through the electrodes on the glass substrate. On this occasion, the palette 41 (and the prober 44) is grounded to be electrically insulated in the high-vacuum chamber.
In the testing apparatus configured as described above, even in the case where the glass substrate of the TFT substrate 11 in the high-vacuum chamber is broken, broken pieces of glass is not scattered over the stage and other devices in the chamber because the broken pieces remain on the palette 41. Hence, the broken pieces can be collected smoothly, so that damage of the apparatus caused by scattering of the broken pieces can be minimized. Further, because the glass portion of the TFT substrate 11 is covered with the prober 44, drive signals can be supplied to the respective pixels 13 while the glass portion is prevented from being negatively charged (charged-up) on the basis of the electron beam 22.
In the testing apparatus configured as described above, there is however a voltage difference between the prober 44 and a pixel 13 driven near the prober 44. Hence, there is a problem that efficiency in detection of secondary electrons emitted from the driven pixel 13 is lowered by the voltage difference. The influence of the voltage difference between the pixel 13 and the prober 44 on the amount of detected secondary electrons will be described below with reference to FIGS. 8, 9A and 9B.
FIG. 8 is a schematic drawing showing the testing apparatus using the pallet and the prober. Specifically, FIG. 8 shows apart of the palette 41 (and the prober 44) of the TFT substrate 11 and its vicinity.
In FIG. 8, the testing apparatus includes a palette 41 (and a prober 44), and drive signal source units 51 and 52 for supplying drive signals to each pixel. The prober 44 is integrated with the palette 41. The drive signal source units 51 and 52 supply drive signals to the source electrode S and the gate electrode G respectively through the prober 44.
In a testing process, the Cs electrode of each pixel and the palette 41 (and the prober 44) are grounded and the drive signal source units 51 and 52 supply drive signals to the source electrode S and the gate electrode G respectively in each pixel. Referring back to FIG. 6, each pixel 13 is driven positively or negatively in accordance with the drive signals, and then the pixel 13 is irradiated with an electron beam generated from the electron beam source 21. Secondary electrons 23 emitted from the pixel 13 irradiated with the electron beam are detected by the secondary electron detector 24 (see FIG. 6). The amount of the secondary electrons detected by the secondary electron detector 24 varies in accordance with the voltage difference between the driven pixel 13 and the prober 44 (and the palette 41) as shown in FIGS. 9A and 9B and as follows.
FIGS. 9A and 9B are schematic drawings for explaining the trajectory of secondary electrons in accordance with the voltage (of each pixel 13) of the TFT substrate 11.
When the TFT substrate 11 is driven positively (e.g., the pixel 13 voltage of the TFT substrate 11 is +5 V) as shown in FIG. 9A, the pixel 13 voltage of the TFT substrate 11 becomes higher than the voltage of the grounded prober 44 (and the palette 41) (i.e., the voltage of the prober 44 is 0 V). Accordingly, secondary electrons 61 emitted from the pixel near the prober 44 rebound from the vicinity of the prober and are effectively detected by the secondary electron detector.
On the other hand, when the TFT substrate 11 is driven negatively (the pixel 13 voltage of the TFT substrate 11 is −5 V) as shown in FIG. 9B, the pixel 13 voltage of the TFT substrate 11 becomes lower than the voltage of the grounded prober 44 (and the palette 41) (i.e., the voltage of the prober 44 is 0 V). Accordingly, secondary electrons 61′ emitted from the pixel near the prober 44 are partially attracted and absorbed to the prober. In this case, the amount of secondary electrons 61′ emitted from a pixel near the prober and detected by the secondary electron detector is smaller than the amount of secondary electrons emitted from a pixel located in the central portion of the TFT substrate 11 and detected by the secondary electron detector. Hence, in a detected image obtained on the basis of the amount of secondary electrons detected by the secondary electron detector, there is a problem that the portion near the prober is mistaken to be a detected pixel, for example, the amount of detected secondary electrons emitted from a pixel near the prober is smaller than a reference value for detection of a pixel defect.